It's the model on which the Tomb of Kings was based on. That particular "dungeon" has been in the game since as long as I can remember (October 23, 1997), but was never used. I was always hoping they'd get around to smashing a hole in one of the walls in Britain (particularly that silly little wall near the Sweet Dreams Inn, just north of the Sewers, that leads absolutely nowhere) or link it to a tomb in the graveyard or some such thing and have something evil lurking beneath Britannia.
Instead, many years later, they modified it, and it became The Tomb of Kings in Ter Mur.
There should, by the way, be one in Felucca's map and one in Trammel's map, as Trammel was just copied from Felucca.
If you've ever gone there in the *coughthingthatshan'tbenamed*, it's basically got a path running around the edges (the big rectangle), the area to the north would seem to be the entrance, and the tunnel that went off to the side (where the secret renowned guy is in Tomb of Kings) actually passes via cave under the external path (the big rectangle). It basically looks like an unfinished dungeon, and I always suspected that something sinister was resting in that tomb off to the side all by itself. Perhaps even Mondain himself.